HS6 carbs + accompanying modifications.

Technical MGB discussion
Ian Fozzard
Posts: 391
Joined: Fri Mar 27, 2015 8:18 am
Forename: Ian
Surname: Fozzard

Re: HS6 carbs + accompanying modifications.

Post by Ian Fozzard » Fri May 24, 2019 8:26 pm

Hi Harry,
I followed the drawings and advice in Peter Burgess's book for most things, doing the cylinder head myself with an air powered die grinder and some carbide burrs. You do need a decent air compressor to run the die grinder, but of course the compressor can be used for spraying and other things. Titrating the head volumes use a burette as found in chemistry labs! You will have to pay someone to skim the head when you know how much to take off of course.
I have done a few heads before so found it straightforward. Ideally I guess it would be best to practice on a spare head if you haven't done it before. Just go very carefully and don't take off too much (disaster if you break into the water jacket!).
I kept the standard distributor set up as I wanted simplicity - carry a couple of spare parts and you are safe! I didn't see the need for anything far away from the standard curve so I just had my original distributor rebuilt by the Distributor Doctor. Max advance set as per Peter Burgess's recommendations.

I went up into the Highlands for a trip yesterday - around 250 miles - and I think I only approached 4500 rpm twice when overtaking . With a speed limit of 60mph generally I am mostly running at under 3000 rpm on an open road - just right for the HR270.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Peter Cresswell
Posts: 613
Joined: Fri Oct 31, 2014 10:05 am
Forename: Pete
Surname: Cresswell
Location: Stone, Staffordshire

Re: HS6 carbs + accompanying modifications.

Post by Peter Cresswell » Fri May 24, 2019 10:17 pm

Over the years I've modified quite a few heads and other parts of the engine on BMC A and B engines, T-Type XPAG, Ford Pinto and pre-crossflow. The key things I've learnt is:
1. Before you do anything make sure the head isn't cracked! most engine machine shops can Magnaflux heads. It only take a few minutes, and will save you a lot of work for nothing. If the head hasn't had unleaded seats fitted do this before you start, but don't get the valve seats cut until you have finished.
2. Without a flow bench don't remove too much metal, particularly in the combustion chambers. Just grind to the bottom of the casting pitting, so the surface is smooth. Too much and you might ruin the gas flow completely. There is a pinch point just inside the inlet ports where you can't take too much metal out or you will break through into the push rod holes.
3. Match the inlet port to the inlet manifold and exhaust port to the exhaust manifold.
4. On MGB heads the 2709 casting with the larger valves and no modification has better gas flow than a stage 1 head with small valves. Don't forget that these valves may need cut outs in the top of the bores for the exhaust valves to open into. On Peter Burgess' web site there are details of how to do this.
5. The greatest gains are to be had from 3 angle seats on both the valve seat and the valve itself, and remove any step on the back of the valve head.
6. On MGB heads, as a rule of thumb, for every 10 thou removed from the head by skimming it reduces the combustion chamber size by 1cc.
7. Don't produce a mirror finish, especially in the inlet ports, as this encourages the emulsified charge to separate out. A rough ground finish prevents this.
8. After the head has been modified do a trial fit using an old head gasket, and turn the engine over by hand to ensure nothing fouls.
9. Reducing the weight of the flywheel will increase the cars acceleration as the car 'appears' to the engine to be much lighter. The B&G road lightweight flywheel has the same effect as making the car over 1 hundredweight lighter, which makes it better value than buying the alloy backplate. Tickover is slightly higher (around 800rpm) though, and when you switch the engine off it will just stop!
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

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